Climate Sensitivity ‘Likely High’

Research in Nature Climate Change claims that climate sensitivity is likely to be relatively high, meaning that future warming will continue in line with high-end projections, contradicting some recent predictions made by the Intergovernmental Panel on Climate Change (IPCC).

The latest IPCC Physical Science report, issued last September, revised its estimate of possible climate sensitivity values, suggesting that very low sensitivities may be possible. Low sensitivity means less warming caused by each ton of CO2 emitted. A Nature Climate Change paper suggests that change may have been a step in the wrong direction.

Dr. Drew Shindell (pictured), a climate scientist at NASA’s Goddard Institute for Space Studies, analyzed results from models that realistically depict aerosols and ozone and found climate sensitivity to be higher.

The simplified calculations that yield low sensitivities do not account for the geographic distribution of ozone and aerosols, instead, they assume these particles are distributed evenly over the globe, which causes the calculations to suggest the observed rate of warming implies low sensitivity, Shindell says.

In the real world, aerosols and ozone are concentrated over industrial areas, primarily the landmasses of the Northern Hemisphere. This is also precisely where they can have the greatest cooling effect because land responds more rapidly to cooling forces than ocean. More realistic models that incorporate this fact show that the observed rate of warming is in fact not consistent with low sensitivity when taking into account the impacts of aerosols and ozone.

Shindell found that the transient climate response is most likely 1.7ºC, and very unlikely to be less than 1.3ºC. This is higher than the IPCC’s low end value of 1.0ºC.

This may seem like a small difference, but sensitivity is important factor for climate projections of the future, Shindell says. Warming would continue even if sensitivity was low, but a high sensitivity means we can expect faster warming as we continue to emit greenhouse gases.